Vibratory feeders and conveyors are known which employ bar shaped leaf springs connecting a trough to a base member. The leaf springs in these feeders or conveyors may be mounted individually, or in banks of multiple leaf springs to meet the spring rate required by the design of the vibratory equipment. The leaf springs are known to be arranged such that one end of the bank of leaf springs is clamped to the conveying member, for example, the trough of the vibratory feeder, and the other end is clamped to the base member, or to the stationary member in the case of a single mass feeder design. In some designs, a center region of the bank of springs is clamped to structure of the conveying member of the feeder or conveyor, while the ends of the bank of leaf springs are clamped to structure of the base member, forming two spring bank sections.
A problem associated with these prior art designs is that as the spring bank is deflected, the leaf springs are required to elongate due to the geometry of the spring bank configuration. This elongation subjects the leaf springs to very high tensile stress as the leaf springs try to stretch. Also, as the feeder operates in each vibration cycle, the leaf springs are required to first deflect, in a characteristic "S" shaped form, in one direction, then to return to pass through a neutral position, and then to deflect in the opposite direction, and then to return to the neutral position once again to complete the cycle. Thus, with each cycle, the leaf springs experience a fully reversing stress which is detrimental to the useful life of the leaf springs.
The generated forces acting along a spring axis are directed to urge the leaf springs in the spring bank to slip in their clamped connection during some stage of deflection. If the clamping force at the clamped connection is increased to prevent such slippage at this stage of deflection, the resulting tensile stress, combined with the increased bending stress of the spring, particularly at the stress riser location formed where the spring is clamped, is often sufficient to cause a premature failure of a leaf spring as it is deflected back and forth.
There have been some prior art attempts to alleviate the design problem discussed in the previous paragraph, by fixing one end of the leaf springs, say to the base member of the conveyor or feeder, and allowing the other end of the spring to rotate. U.S. Pat. No. 3,845,857 discloses an arrangement of a single mass vibratory feeder wherein one end of a spring bank is connected to a rod mounted in an elastomer bushing such that as the spring element are deflected, the bushing yields, allowing the spring ends to move to provide a substantially simple deflection of the spring. This connection avoids the "S" shape form characteristic of deflecting a leaf spring that is fixed at both ends. While this spring mounting means may reduce the spring stresses involved in the deflection, the resultant spring rate would be reduced to an extent that would make the system impractical for large feeders.